Generally, in a multi-layered device for modulating electromagnetic radiation employing an electrochromic material, a physical/chemical change is produced within the electrochromic layer in response to electron or ion transfer caused by an externally applied electrical potential. This change results in modulation of the reflectivity and transmissivity of the device with respect to electromagnetic radiation directed thereagainst. Such devices generally comprise consecutive layers of electrochromic material, an electrolyte-containing fast ion conductor, and a counterelectrode. The exchange of ions between the electrochromic and fast ion conductor layers, when an electrical potential is applied across the device, comprises the mechanism by which the electrochromic layer becomes either bleached (substantially transparent, either lightly colored or colorless) or opaque. By reversing the polarity of the electrical potential applied across the device, it may be "switched" between the bleached and opaque states. Depending upon the magnitude and duration of the applied electrical potential, an intermediate, generally colored, translucent state may be induced, wherein the electrochromic layer contains a concentration of ions sufficient only to reduce the transmissivity of the device but not make it completely opaque to electro-magnetic radiation. Thus, depending upon the manner in which the device is operated, i.e., the polarity, magnitude, and duration of the voltage applied, it may be adjusted to have an electromagnetic radiation transmissivity from 0% to greater than about 90%, with an inversely corresponding reflectivity.
In typical electromagnetic radiation modulating devices, the electrochromic layer comprises an inorganic metal oxide, most commonly a transition metal oxide such as, for example, tungsten oxide. The electrolyte-containing fast ion conductor layer adjacent the electrochromic layer is generally adapted to provide a positively charged light cation such as, for example, a lithium ion. As an example of the operation of a typical electrochromic device, when lithium ions are introduced into a tungsten oxide electrochromic layer, the layer changes from a colorless transparent state to a dark blue-black color; where the tungsten oxide electrochromic layer is sufficiently thick, the induced coloration causes the tungsten oxide electrochromic layer to become opaque to electromagnetic radiation, e.g., the visible portion of the electromagnetic spectrum.
The electrolyte-containing fast ion conductor layer may be a liquid electrolyte solution such as, for example, lithium perchlorate in propylene carbonate; a gel such as, for example, a solution of methanol in polyvinyl butyral doped with lithium chloride; or a solid such as, for example, porous silicon dioxide doped with lithium salts.
Counterelectrodes are generally prepared from a transition metal oxide such as, for example, vanadium oxide or tungsten oxide, or an electroconductive polymer such as, for example, polypyrrole or polythiophene.
In those electrochromic devices generally known in the prior art and discussed hereinabove, the electrochromic layer is the medium which provides the variation in electromagnetic radiation transmissivity and reflectivity; the electrolyte fast ion conductor and counterelectrode layers generally being transparent.
U.S. patent application Ser. No. 07/338,261 to Demiryont discloses an electrochromic device, comprising first and second spaced-apart transparent electrodes and an electrochromic matrix material layer therebetween. The matrix layer comprises a substantially uniform mixture of: a metal salt such as, for example, copper chloride; an ion conductive enhancer such as, for example, lithium nitrate; and an ion conductive material such as, for example, polyvinyl butyral gel. Although the electrodes are transparent, the electrochromic device appears light yellow in transmitted color due to the color of the metal salt which is in solution in the matrix layer. When an electrical potential is applied across the electrodes, the metal atoms of the metal salt plate onto the cathode while the metal salt anions migrate toward the anode. In this state, the electrochromic device may be opaque to electromagnetic radiation, depending upon the thickness of the metal layer formed at the cathode. The device, however, has what is known in the electrochromics art as a "short term memory." The matrix layer readily reverts to its initial state, i.e., the metal layer redisolves back into the matrix layer which takes on a light yellow color in transmitted light, when the electrical potential is removed from the electrochromic device. It is further disclosed that a counterelectrode may be interposed between the matrix layer and the anode. This counterelectrode minimizes the formation of a gas at the anode caused by the migration of anions toward the anode where they are oxidized, such as occurs when copper chloride is used as the metal salt resulting in the production of chlorine gas at the anode.
U.S. Pat. No. 4,256,379 to Green discloses an electrochromic device comprising consecutively: a first electrode such as, for example, tin oxide coated glass; a metal sensitive colorable electrochromic material such as, for example, tungsten oxide; a metal ion-containing fast ion conductor such as, for example, rubidium silver pentaiodide; and a second electrode such as, for example, silver. The metal ion of the fast ion conductor is effective to color the electrochromic material layer when injected therein. The second electrode must be formed from a material containing metal identical to the metal ions of the fast ion conductor. Thus, in a preferred embodiment, when an electrical potential is applied across the device, silver ions from a silver second electrode are injected into a rubidium silver pentaiodide fast ion conductor, while other silver ions from the fast ion conductor are injected into a tungsten oxide electrochromic material causing it to turn blue. It is disclosed that the second electrode can be very small, such as a Dag contact placed on the surface of the fast ion conductor layer (as opposed to a continuous film adhered to the fast ion conductor), and serves merely as a source of the fast metal ions. The second electrode does not participate in the modulation of the transmitted or reflected electromagnetic radiation.
It would be desirable to prepare an electromagnetic radiation modulating device, wherein modulation of the transmissivity and reflectivity of electromagnetic radiation could be precisely controlled over a wide range. Such a device would be particularly useful were it able to substantially reduce the transmission of infrared radiation as well as visible light rays. Thus, the device could be used to prevent the passage of heat energy therethrough, and would therefore be especially suited for use as an automotive or architectural glazing. Furthermore, the usefulness of such a device would be particularly enhanced were it able to maintain an established transmissivity or reflectivity after the removal of an electrical potential.